Flashlamps are used in many different applications including photocopiers, lasers, analytical and clinical instruments, machine vision strobes, aviation obstruction warning beacons medical devices and instruments to monitor the launch characteristics of objects including golf balls. A flashlamp is an arc lamp that operates in the pulsed mode and is capable of converting stored electrical energy into intense bursts of radiant energy covering the ultraviolet (UV), visible (VIS), and infrared (IR) regions of the spectrum. Flashlamps are similar to all other arc lamps in that optical radiation is produced by passing an electrical current through a gas. Both continuous and line spectra are produced when sufficient energy is transferred to the gas atoms to cause excitation and ionization. See "Transport Phenomena in a Completely Ionized Gas", Spitzer, L., Jr., and R. Harm, Phys. Rev., 89, 977-981. (Spitzer).
Xenon is used in most flashlamps, since it is the most efficient of the inert gases at converting electrical energy to optical energy. Krypton is sometimes used because of its high efficiency in the near-infrared region. Current is usually supplied by a charged capacitor capable of discharging large amounts of energy in a short period of time. Depending on the value of the capacitor and other circuit components, pulse widths from under 1 microsecond to over 50 milliseconds and energies from millijoules to kilojoules can be produced.
This characteristic of producing energy in pulses of extremely high peak values and very short duration is a great advantage in many applications because it means that while peak optical power is typically in the kilowatt-to-megawatt region, the average power, which determines the size and cost of the power supply is in the watts-to-hundreds-of watts region. A further advantage of flashlamps is that even though the output spectrum is broadband, specific regions of the spectrum can be emphasized by controlling current through the flashlamp. For instance, at relatively low values of lamp current the spectral output is heavily weighted toward the visible and infrared end of the spectrum. As current is increased, the output shifts toward the blue and ultraviolet. See Design Considerations for Triggering of Flashlamps, Alex D. McLeod, EG & G Electro-Optics, Salem, Mass. November 1996 ("McLeod").
FIG. 1 provides an illustration of a flashlamp circuit comprising a power supply, a capacitor 1 and a flashlamp 2. In the nonionized state, a flashlamp has high impedance (tens of megohms). Therefore, all current from the power supply initially flows into the capacitor 1 as shown by the circuit of FIG. 1. As the voltage across the capacitor 1 is increased, a point is reached, called the breakdown voltage, where xenon atoms are ionized and the impedance of the flashlamp 2 starts to drop. In a short period of time, enough xenon atoms are ionized so that a low-impedance path is formed from anode to cathode, and current flows from the capacitor 1 through the flashlamp 2. As this occurs, more xenon atoms are ionized and the arc impedance continues to drop to the milliohm region. The arc also expands outward to eventually fill the bore of the flashlamp. Most of the energy stored in the capacitor 1 is expended in a matter of microseconds so that, eventually, the current through the flashlamp 2 drops to such a low level that the tube deionizes and stops conducting. At this point, the capacitor 1 starts recharging. See Linear Flashlamps Technical Brief, E6&G Electro Optics Salem, Mass., 1995. (Linear Flashlamps).
Although the circuit in FIG. 1 is a useful illustration of how a flashlamp works, it is not a practical circuit. The breakdown voltage of most xenon flashtubes is high, typically 10 kV or more, and is not very repeatable. Therefore, most practical flashlamp circuits utilize a capacitor charging voltage which is much lower than the breakdown voltage. Conduction is then initiated by application of a brief high-voltage trigger pulse. Previous approaches have used different types of triggering to activate the flashlamp including: external, series, pseudo series, simmer, and pseudo simmer. See McLeod, pg. 2-8.
FIG. 2 provides an illustration of an external trigger comprising a capacitor charging power supply, capacitors 4, 8, an inductor 5, a flashlamp 6, a transformer 7 a resistor 10 and a silicon controlled rectifier (SCR) 9 having a trigger pulse input 11, external triggering creates a small arc streamer between the electrodes by applying a high voltage trigger pulse to a thin wire wrapped around the outside of the flashlamp 6. The pulse can also be applied to a metal bar, reflector, or cavity as long as the metal covers the entire distance between the electrodes. In these latter cases, the spacing between the lamp and metal piece should be no more than 1/4-inch and somewhat higher trigger voltages may be needed. The trigger pulse 11 is supplied by a high-turns ratio transformer 7 which can be compact and lightweight, since it has to produce high voltage but little current (100-300 mA). A finite amount of time is required for the trigger streamer to propagate down the bore of the flashlamp. The pulse duration for external triggering should be about 200 nanoseconds per inch of arc length. Required trigger voltages depend on arc length, bore size, fill pressure, and electrode material and are typically listed in flashlamp specification sheets. See McLeod, pg. 2-3.
In the series technique, the trigger voltage is applied directly to one of the flashlamp electrodes from the secondary of a transformer which is placed in series with the flashlamp. Again, the purpose is to create a small arc streamer between the electrodes. Although it is not required, the trigger wire, used for external triggering, may be left wrapped around the lamp and grounded. This facilitates triggering by lowering the voltage requirement. The series trigger transformer is larger and heavier than the parallel transformer since the secondary must carry the full flashlamp current. Also, the secondary adds impedance to the circuit, and this must be considered in the circuit design. In fact, by choosing a trigger transformer having the proper value of saturated inductance, no other choke should be necessary to achieve critical damping. The trigger pulse duration for a series trigger is 150 nanoseconds per inch of are length, and the required voltages are typically listed in flashlamp specification sheets. See McLeod, pg 4-6.
In the pseudo-series technique as in the series technique, the trigger voltage is applied directly to one of the flashlamp electrodes. However, in this circuit an external trigger transformer is used but the capacitor discharge does not pass through the transformer secondary. Blocking diodes prohibit the trigger voltage from appearing across the discharge capacitor. The blocking diodes must be selected with care as they not only hold off the high voltage trigger pulse but must be capable of carrying the discharge current as well. Should critical damping be required, an inductor of appropriate value must be added in the discharge loop. See McLeod, pg 6-7.
The simmer mode technique utilizes a separate power supply to maintain a continuous DC current through the flashlamp and keep it in the ionized state. Typical simmer currents are 100 milliamps up to several amps. Flashlamp pulsing is accomplished by closing a switch, typically a silicon controlled rectifier (SCR), in series with the capacitor and flashlamp. An external or series trigger circuit is also required to initially start the flashlamp. In the pseudo-simmer circuit, the simmer current is turned on just before the main discharge so the flashlamp is pre-ionized. See McLeod, pg 7-8.
U.S. Pat. No. 4,742,277 discloses a pulse generating apparatus including a base current supply section for generating a constant dc current having a first prescribed current level for turning on a xenon lamp, and a pulse current section for adding a pulse current having a second current level greater than the first current level and a prescribed pulse duration within a prescribed repetition period to the constant dc current.
U.S. Pat. No. 5,196,766 discloses a discharge circuit and a method of operating a flashlamp wherein the flashlamp is reliably operated repetitively while reducing current surges from the electrical power source. The circuit provides a switch means for shunting recharge energy through a non-reactive means around an energy storage means for the flashlamp. The rate of recharging the energy storage means is reduced at the beginning of recharging below the rate which would allow the flashlamp to conduct before intentional triggering of a flash.
The activation of flashlamps must consider peak current and recharge time. It is important to know the peak current through a flashlamp for several reasons. First, the spectral output is a function of the current density through the flashlamp. As the current density is increased from a few tens of A/cm.sup.2 (cross-sectional area) to a few thousand A/cm.sup.2, the intensity in the blue and ultraviolet increases far more rapidly than the red and infrared. Color temperature at the low currents is about 5000.degree. Kelvin and at the high currents about 10,000.degree. Kelvin. In addition, the line structure, which is strong in the infrared at low current densities, becomes almost completely masked at high current densities. At current densities under 4000 A/cm.sup.2, both xenon and krypton have a particularly good match with the absorption curve of neodymium-YAG laser material. Further, there are limitations on peak current which, if surpassed, result in damage to the lamp and early failure. See Linear Flashlamp pg. 5-6.
The maximum flashing frequency is determined by the deionization time of the flashlamp and the charging rate of the capacitor. If the charging current rises too quickly after the lamp has flashed, it will go into a holdover condition. In the holdover state, the flashlamp never deionizes. Instead, the charging current bypasses the capacitor and flows through the lamp in a continuous DC arc. Holdover can permanently damage a lamp in a very short time because of overheating of the electrodes. Holdover actually occurs when the capacitor voltage rises to the minimum tube operating voltage before deionization occurs. See Linear Flashlamps, pg. 8.
While previous research has disclosed different methods for activating a flashlamp, there remains a need for a system which achieves fast sequential strobing with a single lamp while limiting the charging current to prevent holdover and while limiting the peak current to the flashlamp to extend the life of the lamp and to weigh the spectral output heavily toward the visible and infrared end of the spectrum.